1. Field of the Invention
The present invention relates to an aqueous binder, which is crosslinkable at room temperature, and its use as a coating agent and for binding solid objects.
2. Discussion of the Background
Paint technology has always used liquid coating agents containing a binder which dries at room temperature. Classic oil-based paints contain binders in the form of drying oils which under the influence of oxygen in the air harden due to the radical polymerization of unsaturated groups in the oil molecules. The evaporation of the solvent and simultaneous crosslinking results in the formation of a hard, dry surface with a high solvent resistance.
For reasons of environmental protection, there is a growing interest in replacing solvent-based coating agents with aqueous-based, solvent-free systems. But the quality of the dried coatings produced with such solvent-free systems does not correspond to that of the classic paints. The aqueous coating agents contain latex-like binders which form a film during drying, but do not crosslink. The obtained coatings are frequently soft and sticky (tacky) and are susceptible to organic solvents.
There has been no lack of attempts to produce aqueous coating agents which harden into hard, dry films while crosslinking. As an example, self-crosslinking plastic dispersions containing emulsion polymers which contain N-methylol groups and possibly additional dissolved aminoplast resins are widely used. But these only harden at increased temperatures and, thus, are called aqueous baking lacquers.
The objective of a binder which crosslinks at room temperature or slightly above is hard to realize, since hardening starts even before film formation during the storage of the crosslinkable coating agent. To circumvent this problem, two-component systems in which binder components able to react at room temperature are mixed only just before forming a coating film so that they are able to harden in the form of a coating film are frequently used. The use of two-component coating agents requires special coating equipment, such as two-component spray guns with accurate metering devices for both components.
EP-A 184 091 describes aqueous binders for paints which are characterized by a low film forming temperature and high block resistance of the formed films. This binder is characterized by a multi-stage emulsion polymer, the first stage of which yields a relatively soft polymer core and the second stage of which yields a significantly harder polymer shell of the latex particles. For the purpose of crosslinking, the emulsion polymer may contain units of unsaturated carbonyl compounds, such as diacetone acrylamide which are reacted with dihydrazides. The adhesion of the coating films on the coated substrates furthermore can be improved if the emulsion polymer also contains units of unsaturated ethylene urea derivatives. Emulsion polymers containing both units of unsaturated carbonyl compounds and units of ethylene urea derivatives have not been described.
Aqueous plastic dispersions containing alkylene urea groups are known. According to EP-A 379 892, these groups are used to improve the adhesion of films formed from the dispersions on solid substrates, or, according to EP-A 384 125, as crosslinking-reactive group and simultaneously as formaldehyde getters in self-crosslinking plastic dispersions containing N-methylolamide groups. But it was not known that alkylene urea groups could be crosslinked by means of aldehyde functional groups.
Thus, there remains a need for aqueous-based, solvent-free binder composition which are crosslinkable at room temperature or temperatures slightly above room temperature.